The present invention relates to an electrically conductive polymer composition formed from a solid solution of a dissociable salt in a polymer. Electrically conductive polymers of this type, herein referred to as ion conducting polymers are employed in numerous applications such as energy storage, sensors, selective membranes, antistatic films, and electrochromic devices. Typically, such applications dictate that the conducting polymer composition exhibit a conductivity of at least 10.sup.-6 S/cm (Siemens per centimeter) at room temperature (25.degree. C.). When applied in electrochromic devices, such polymer systems must also have good optical properties and be easily processable, particularly for lamination operations.
Ion conducting polymer compositions at a minimum comprise one or more polymers and an effective amount of at least one dissociable salt dissolved therein. The conductivity of the resulting composition appears to result from the presence of the solvated salt, while the polymer provides a stable network for maintaining solvation of the salt. Thus, the conductivity of the composition depends upon the solvation capacity of the polymer, temperature, amount of salt dissolved therein, and form of the network structure.
The reason for differences in solvation capacities between polymers stems from the multidentate nature of intramolecular solvation and the tendency for some unhindered, long chain molecules having donor electrons to adopt a cage configuration in which the donor electrons are cation. This solvation mechanism is well known for crown ethers in which cages of predetermined size are employed to specifically solvate cations of corresponding radius. The prior art has recognized that a polyethylene oxide (PEO) based ion conducting polymer composition appears to offer the best combination of a high density of donor heteroatoms (i.e. the oxygen atom of PEO) and a sufficiently long linear hydrocarbon chain segment
In most ion conducting polymer compositions, in order to achieve relatively high conductivities (i.e. 10.sup.-6 S/cm) the polymer must be in an amorphous state to sufficiently solvate the salt. Thus, it is necessary to suppress the crystallization of the PEO-based polymer such that the polymer is in an amorphous state at room temperature. One method of obtaining an amorphous polymer composition at room temperatures is to utilize a short chain, low molecular weight polymer. However, such short chain polymers do not sufficiently possess mechanical integrity (may even be a paste or a liquid) at room temperature. Therefore, other means must be utilized to suppress the crystallization of the PEO-based polymer while retaining relatively long chain segments in the polymer.
There are other methods which are available to suppress the crystallization of a polymer. These include disrupting the polymer chain to minimize its tendency to crystallize. This may be done by introducing comonomers, adding side chains, utilizing a random or block copolymer or cross-linking of the polymer. Plasticizers may also be added to the polymeric composition to modify the polymer's T.sub.g. Although the use of ion conducting polymers in electrochemical devices has been recognized by the prior art, and the application of such polymers to electrochromic devices has been envisaged, this approach represents a unique method to achieve an ion conducting polymer composition having a conductivity of at least 10.sup.-6 S/cm in conjunction with high optical quality and processing characteristics to enable use of the composition in electrochromic devices.